Researchers at the University of British Columbia have uncovered a groundbreaking discovery that challenges the long-held belief in genetics known as the “one nucleus, one full genome” rule. Their study reveals that certain fungi possess a unique genomic structure, with genomes split across multiple nuclei. This finding has implications for our understanding of eukaryotic organisms, which include plants, animals, and fungi.
Genomes serve as the essential blueprint for living organisms, containing all the genetic information required for their growth and maintenance. In most eukaryotes, genomes are housed within the nucleus and organized into chromosomes. For example, the human genome is structured into 23 chromosomes. Until now, it was presumed that each nucleus contained a complete set of chromosomes. However, this new research demonstrates that in two species of fungi, the genome is divided between multiple nuclei, with each nucleus only holding part of the total chromosomes.
Revolutionary Findings in Fungal Genetics
The researchers focused on the fungus Sclerotinia sclerotiorum, a soil-borne pathogen responsible for stem rot and white mold in crops such as canola, soybean, and sunflower. Despite its agricultural significance, the genetics of S. sclerotiorum have remained poorly understood. During their investigation, the research team made a surprising discovery about the behavior of the fungus’s chromosomes during reproduction.
Typically, eukaryotic cells are diploid, meaning they contain two copies of each chromosome. In many fungi, reproduction begins with a diploid parent cell dividing to create haploid spores, each with a single nucleus housing one set of chromosomes. However, S. sclerotiorum produces ascospores, which contain two separate nuclei. Initially, it was thought that each nucleus held a haploid set of chromosomes, suggesting that an ascospore contained a total of 32 chromosomes.
Through advanced fluorescent microscopy, the researchers directly counted the chromosomes in a single ascospore and consistently found only 16 chromosomes, contradicting the previous assumption. Further analysis using fluorescent probes revealed that the two nuclei within each ascospore contain distinct chromosomes.
Insights into Genomic Division
The team explored whether the distribution of the 16 chromosomes between the two nuclei was random or followed a specific pattern. By isolating individual nuclei and employing polymerase chain reaction (PCR) analysis, they discovered that the composition of chromosomes varied among the nuclei, indicating an irregular division of chromosomes.
Curious about the broader implications of their findings, the researchers turned their attention to another related species, Botrytis cinerea. This fungus, also a plant pathogen, typically produces spores with four to six nuclei. Using similar methodologies, the team found that the 18 chromosomes in B. cinerea’s genome are also divided among its nuclei, with each nucleus carrying from three to eight chromosomes. This suggests that the phenomenon of haploid genome division across nuclei may extend beyond S. sclerotiorum to other fungi.
The implications of this research raise questions about the mechanisms fungi use to ensure genetic integrity during reproduction. In order to generate a new generation, these fungi must reform a diploid cell with a complete set of chromosomes, likely requiring the fusion of nuclei with complementary chromosomes. The researchers speculate that a selection process may exist, where only nuclei that together form a complete genome lead to viable ascospores.
This research offers a new perspective on fungal life cycles and the dynamics of nuclei. The findings may also pave the way for advancements in gene editing, enabling researchers to manipulate chromosomes and nuclei more effectively.
As they continue their investigation, lead researcher Xin Li, alongside colleagues Edan Jackson and Josh Li, hopes to uncover further insights into these fascinating mechanisms, broadening our understanding of genetic organization in fungi and potentially other eukaryotes. This work not only challenges existing genetic paradigms but also holds the promise of transformative applications in biotechnology and agriculture.
